1. Field of the Invention
The present invention relates to a vibration isolator for isolating vibration in an apparatus, and more particularly, to a vibration isolator for isolating vibration and a method of isolating vibration in an exposure apparatus used in photolithography processes for manufacturing semiconductor devices or the like.
2. Discussion of the Related Art
The manufacture of semiconductor devices (or liquid crystal display devices, imaging devices such as CCDs, thin film magnetic heads, or the like) employs photolithography in order to expose a pattern on a reticle (or photomask or the like) onto a wafer (or a glass plate or the like). In the photolithography process, projection exposure apparatus such as a stepper, proximity-type exposure apparatus, and the like have been used. In these exposure apparatus, an extremely high degree of vibration isolation is required in order to transfer the pattern of the reticle onto the wafer with high alignment precision. To this end, the main body of such an exposure apparatus (the portion that transfers the pattern of the reticle onto the wafer) is placed on a vibration isolator which suppresses vibration having external and internal origins.
For example, a one-time exposure type exposure apparatus, such as a stepper, which transcribes a reticle pattern onto a wafer with high alignment accuracy, is required to have highly advanced vibration isolating characteristics.
Recently, a scanning exposure type projection exposure apparatus, such as step-and-scan system, has drawn a great deal of attention. Such a scanning exposure apparatus allows a larger reticle pattern to be transcribed onto a wafer by exposure without enlarging the projection optical system. In this system, a reticle pattern is successively transcribed onto a wafer by exposure in synchronization with reticle movement in the direction perpendicular to the optical axis of the projection optical system. In this case, the wafer moves in a predetermined direction at a speed corresponding to the magnification of the projection optical system. These scanning exposure type projection exposure apparatus require stable movement (scanning) of the reticle and the wafer at constant speeds. Therefore, external and internal vibrations must be eliminated via vibration isolators.
A vibration isolator is usually composed of spring-like materials and vibration damping materials. In the conventional vibration isolators, the performance in vibration isolation does not depend on the vibration status or the position (posture or the like) of the apparatus. Such vibration isolators are generally called "passive vibration isolators". In related work, "active vibration isolators" have recently come into use. The active vibration isolator detects the external and internal vibration in real time using sensors, such as an accelerometer (acceleration sensor), displacement sensor, or the like, and changes its performance according to the detected results.
The following two functions are primarily required of vibration isolators used in exposure apparatus. The first function is to block the transmission of vibration originating from the floor, where the exposure apparatus is placed, to the main body of the exposure apparatus. The second is to attenuate the vibration of the body of the exposure apparatus generated by internal motions of the apparatus, such as wafer and reticle stage motions or the like. In passive vibration isolators, these two functions have an inverse relationship with each other--when one of the functions is strengthened, the other is weakened. In contrast, active vibration isolators are capable of satisfying the two functions above and hence have garnered much attention recently.
An active vibration isolator in general uses an air spring (air damper) as a support mount (vibration damping mount) for supporting the weight of the apparatus main body which is an object of vibration isolation. The air spring is a soft spring having small rigidity, and thus, blocks the transfer of the middle and high frequency components of vibration (20 Hz and up) out of the entire vibration components coming from the floor. The active vibration isolator also employs vibration detection sensors and actuators to suppress the low-frequency-range components of the vibration using feedback control.
In more detail, when applied to an exposure apparatus, the active vibration isolator of this type is usually composed of 1) support mounts for supporting the weight of the exposure apparatus body; 2) position sensors for detecting the posture and position of the exposure apparatus body; 3) vibration sensors (velocity or acceleration sensor) for detecting the motion status (vibration) of the exposure apparatus body; 4) actuators for generating forces to suppress the vibration of the exposure apparatus body; and 5) a control unit for calculating thrusts to be generated by the actuators according to the results detected by the sensors. In order to stabilize and support the exposure apparatus body and to measure and control the vibration of the exposure apparatus body for all degrees of freedom (six degrees of freedom), the conventional active vibration isolator needs to employ at least three or more support mounts, six or more position sensors or vibration sensors, and six or more actuators.
In related work, packaged active vibration isolators have been announced. These packaged active vibration isolators can be applied directly to exposure apparatus without modifying the exposure apparatus. However, it is more efficient from space considerations to use vibration isolator that is made into one unit with the exposure apparatus body.
As noted above, in such an active vibration isolator that is incorporated into the exposure apparatus body, it is necessary to install a large number of constituent elements in order to efficiently suppress vibration for all six degrees of freedom (three degrees of freedom for the translational direction and three degrees of freedom for the rotational direction). In particular, when detecting rotational vibration which has three degrees of freedom, the vibration component for each degree of freedom is obtained by using two detection sensors measuring the translational vibrations at their respective positions remote to each other, and by extracting the difference between the measured values of the two detection sensors. To obtain high precision in such measurement, the spacing between the two detection sensors must be as large as possible. This spacing requirement between the detection sensors creates a problem.
In addition, in the case of precision vibration isolators, such as those used in exposure apparatus, forces (reaction forces) are exerted in the vertical direction to the exposure apparatus body by three or more support mounts and by a plurality of actuators (also referred to as "Z actuators") which generate thrusts in the vertical direction. Conventionally, the support mounts and Z actuators are positioned in parallel between the installation floor and the mounting plate of the exposure apparatus body. As a result, the reaction forces of these support mounts and Z actuators may elastically deform the mounting plate of the exposure apparatus receiving these forces. This prevents the precise positioning of the wafer and reticle stages, for example. To solve this problem, the following vibration isolators have been proposed.
(A) Elastic deformation is reduced by strengthening, to the extent possible, the rigidity of the mounting plate which receives forces from the Z actuators and support mounts.
(B) The Z actuators are installed inside the exposure apparatus body (target object for which vibration are to be reduced) instead of between the floor surface and the exposure apparatus body. Vibration of the apparatus are suppressed using inertial forces generated by such Z actuators.
(C) The support mounts themselves generate thrusts in the Z direction. Z actuators are not provided in particular.
However, in the system (C), a new technology is needed in order to solve the problems of heat generation and poor durability of the support mounts. In the system (A), it is impossible to cope with precision exposure apparatus where even minute deformations of the mounting plate are not permissible. There is also a concern that the mounting plate will become excessively large in this case. In the system (B), there arises a problem that the vibration reduction efficiency is small and the production cost would become very high in order to achieve sufficient vibration reduction effect.
Recent exposure apparatus requires much higher accuracy in positioning of the wafer and reticle stages or the like than the conventional exposure apparatus. Also, because the structure of the exposure apparatus is becoming more complicated, the residual vibration amount allowed in an active type vibration isolator is becoming smaller--from the micron order to the sub-micron order. In order to perform such high precision control, it becomes necessary for the control unit described in (5) above to obtain the mechanical constants, such as the weight, center of gravity, moment of inertia, and the principal axis of inertia, with considerable accuracy
However, in the case of projection exposure apparatus such as a stepper, it has been thought to be nearly impossible to obtain sufficiently accurate mechanical constants using a simple method. One method that has been used is to calculate the mechanical constants with reference to the design drawings. The accuracy of the mechanical constants estimated in such a way can be improved by measuring mechanical constants or the like of each part of the apparatus. However, the accuracy is still not sufficient, and a problem occurs when such calculated constants are used in the control unit as mechanical constants of the entire apparatus, because the vibration of the apparatus cannot be controlled with a high degree of accuracy.
Furthermore, the conventional vibration isolators have the following problems. The active vibration isolator generally employs a vibration damper for the vibration isolating bed (which supports the weight of the entire exposure apparatus) to cut off vibration components at the high-frequency band. The vibration isolating damper, made of air spring (air damper), exhibits softness and low rigidity. Although such a vibration damper cuts off vibration components at the high-frequency band, it does not suppress much vibration at the low-frequency band because of low attenuation. Such a vibration damper alone cannot quickly suppress low-frequency vibration. Therefore, ordinary active vibration isolators use additional air pressure type actuators (pneumatic actuators driven by air pressure) to suppress low-frequency vibration, in conjunction with the above-mentioned vibration damper.
Nevertheless, since these air pressure type actuators have a response frequency as low as 10 Hz, the vibration isolator for use in the scanning exposure apparatus that has movable stages or other parts moving at high acceleration rates cannot produce sufficient vibration isolating effects. Accordingly, vibration isolators for use in such an exposure apparatus employ electromagnetic actuators (driven by electromagnetic force), which feature superior response rates for suppressing high-frequency vibration. The electromagnetic actuators can operate at a response speed higher than or comparable to the acceleration and deceleration rates of the movable stages or other parts.
However, if this electromagnetic actuator is used to suppress vibration at a wide frequency range, from low to high frequencies, a large quantity of heat is generated from the actuator's coil portion. This is because the actuator must let a current flow constantly to suppress vibration components at extremely low frequencies, which can be regarded as offset components (displacement or rotation at zero frequency). Such offset components are caused by inclination of the exposure apparatus that may occur due to relocation of the apparatus, inclination of the exposure apparatus due to inclination of the floor on which the apparatus is installed, or rotation of the exposure apparatus accompanying the relocated wiring and piping around the apparatus. Measuring instruments, such as laser interferometers, are installed to measure the positions of movable stages in the apparatus. A laser interferometer determines the position of the stage by radiating a laser beam to a mobile mirror fixed on the stage and receiving the laser beam reflected from the mirror. Therefore, if a large quantity of heat is generated from the electromagnetic actuator, the air on the optical path of the laser beam fluctuates, thereby decreasing the interferometer's measuring accuracy.
In recent years, ever-higher positioning accuracy becomes necessary in an exposure apparatus due to the increasing integration for VLSI, etc. In response to this trend, there has been a high demand for active vibration isolators that can suppress vibration of the exposure apparatus both in translational and rotational directions.